WASHINGTON (Reuters) - Standing in front of a memorial made of remnants of the World Trade Center, U.S. President Donald Trump will have a powerful symbolic opportunity in Brussels next Thursday to make clear how he really feels about NATO.

But as controversies swirl around Trump over alleged ties between his 2016 campaign and Russia, it is unclear whether the Republican president - who slammed the post-World War Two military alliance as "obsolete" when he was running for office - will say the words that whipsawed NATO partners really want to hear.

Alarmed by Russian aggression in Ukraine and wary of the U.S. administration's efforts to build friendlier ties with Moscow, European partners want to know if they have Trump's staunch support.

While the White House says Trump will reaffirm the U.S. commitment to the alliance in his remarks at NATO, allies also want a full-throated endorsement of Article 5 of the treaty - the principal that an attack against one member is treated as an attack against all.

The 28-member North Atlantic Treaty Organization, which was formed in 1949 during the Cold War, has invoked the collective defense article only once - after the Sept. 11, 2001, attacks on the United States that leveled the World Trade Center's twin towers in New York, Trump's hometown.

Trump plans to dedicate a memorial to the invocation of Article 5 at the new NATO headquarters.

"The problem in the plan is that President Trump is the only president who has not yet explicitly endorsed Article 5," said Thomas Wright of the Brookings Institution think tank. "I understand that is not an accident."

On the campaign trail, Trump accused NATO allies of not paying their fair share for defense and not focusing enough on the fight against terrorism.

Since last November's election, Trump and his aides have tempered those remarks. U.S. Defense Secretary Jim Mattis - who will travel with Trump to Brussels - has pledged support to Article 5, as has Vice President Mike Pence.

Trump has said publicly his views on NATO have changed, telling reporters during a White House visit with NATO's Secretary General Jens Stoltenberg that NATO was "no longer obsolete."

"He has never said, like Pence, like Mattis, that U.S. commitment to Article 5 is rock solid, and allies want to hear that at this summit," said Julie Smith, national security aide to former Vice President Joe Biden.

POLITICAL UPROAR AT HOME

While abroad, Trump will be shadowed by the political tumult that ensued after his firing earlier this month of FBI Director James Comey. The U.S. Justice Department has named a special counsel to probe possible collusion between Trump's campaign and Russia, which Trump denies. He said on Thursday he was the victim of a political witch hunt.

Trump, who took office in January, departs on Friday for his first foreign trip as president and will visit Saudi Arabia, Israel, the Vatican and Sicily in addition to Brussels.

A senior White House official said Trump would press his demands for NATO partners to step up their defense spending.

The message may undercut any effort to offer reassurance to NATO allies about the commitment to the alliance and to Europe, said Wolfgang Ischinger, a former German ambassador to the United States and Britain.

Republican Senator John McCain, chairman of the Senate Armed Services Committee and a frequent Trump critic, expressed frustration with the president's NATO stance.

Asked by a reporter what he would like to see Trump do to reassure NATO allies, McCain laughed, saying: "Talk about his commitment to NATO - that would be an opener."

(Additional reporting by Noah Barkin in Berlin and Patricia Zengerle and Yegeneh Torbati in Washington; Writing by Roberta Rampton; Editing by Caren Bohan and Peter Cooney)